Field of the Invention
The invention concerns a termination unit for a single- or multiphase superconducting cable.
Description of the Related Art
Single- or multiphase superconducting cables consist: of one or N concentrically arranged superconducting phase conductor(s) and an external neutral or screen conductor around a tubular or massive supporting core extending along a longitudinal axis, whereby all the superconducting phases as well as the outermost superconducting phase and the neutral conductor are separated from each other by an insulating layer. Each superconducting phase conductor may consist of multiple layers of superconducting wires or tapes, wherein an electrical insulation might be present in between the layers of one phase conductor. The cable is arranged within a thermally insulating envelope. The thermally insulating envelope as well as the tubular supporting core may contain a refrigerating fluid, such as liquid nitrogen. Generally, multiphase superconducting cables are three-phase cables (N=3). They are, for example, used as power cables installed in an electrical grid.
Termination units for single- or multiphase superconducting cables are needed in order to connect to cable to the grid, i.e., for managing the transition between the cooled, superconducting regime and the normally conducting regime at ambient temperature. An example of such a termination unit is described in the European patent application EP 2 523 290 A1. This termination unit comprises cylindrical modular elements, one for each phase, each modular element having an external thermally and electrically insulating envelope. The external envelopes contain portions of the superconducting cable, which themselves are contained in internal envelopes made from a thermally and electrically insulating material and filled with cooling fluid. Branch elements of the phases are arranged at the ends of the modular elements and directly connected to the cable portions, and only electrical connection elements, e.g. lugs, of the branch elements project from the modular elements.
However, such a termination design implies external electrical fields present at the outside of the termination unit. These external electrical fields are difficult to manage if the termination unit is to be used, for example, in a substation. Indeed, in this case, additional measures and costs have to be engaged in order to ensure the security of the operator.
WO 03/034447 A1 is concerned with a superconducting cable termination for a superconducting cable which is surrounded by a termination conduct. A cold section is part of the conduct which is filled with a coolant. The following section is a transition duct which is followed by an ambient temperature section. This section contains a gaseous coolant at conduct pressure and ambient temperature. Internal connections and external connections are located in the ambient section.